1. Field of the Invention
This invention is related to mechanisms which compensate for temperature varying load cell inaccuracies. In particular, this invention directs itself to a load cell system mounted on an external member to which is applied a force load. More in particular, this invention relates to a mechanism for maintaining a substantially constant reference load force on the load cell independent of temperature variations applied to the external member. Further, this invention directs itself to a load cell temperature compensation system which allows for expansion or contraction of load measuring elements in a predetermined manner to relieve the load cell of inaccuracies associated with temperature variation.
2. Prior Art
Temperature compensation systems are known in the prior art. The best prior art known to the inventors include U.S. Pat. Nos. 3,241,359; 3,864,966; 2,930,227; 2,298,216; 4,325,048; 2,357,356; 3,290,928; 3,351,880; 3,341,796; 3,290,931; 3,171,276; 3,108,470; 2,531,414; and, 3,577,779.
U.S. Pat. No. 3,241,359 is directed to a thermally balanced load cell device. The load cell of this Patent has a predetermined structure to minimize thermal errors resulting from a change in temperature. Although this Patent does recognize the problem areas associated with temperature changes in load cells, it does not provide for a mounting system adaptable to a slave type load cell as is provided in the subject invention concept.
U.S. Pat. No. 3,864,966 is directed to a load transducer which compensates for temperature variance. Both the load member and the thermal compensation bar in this Patent are generally constructed of the identical materials in order to reduce thermal variance errors. Once again, this does provide for the general recognition of the problem of temperature variance, however, it does not provide for the mounting mechanism adaptable to load cells of the subject invention concept.
In many prior art load sensing devices which are used for determining the load applied to the end of a boom crane, have proven substantially unreliable and generally provide for load measuring inaccuracies due to temperature induced errors. In some of the prior art crane load sensing devices, such inaccuracies in some cases have merely been treated by downgrading the capacity of the crane to insure safe margins of operation for the lifting capabilities of the crane systems. However, the mere downgrading of the capacity of the crane systems does not provide for an accurate determination of the force loads applied.
The use of slave load cells for measuring deflections by mechanically entrapping a particular load cell between two reference points of a load bearing structure has a number of advantages which include, but are not limited to, the ease of attachment of such slave loading cell systems to existing hydraulic cylinder rods. In general, such slave load cell systems may be bolted directly on the hydraulic cylinder rods at two places, with the load sensing element between the two points of mounting. However, prior art systems directed to temperature compensation of such load sensing systems were not attainable, due to the effects of varying temperatures and the difference in temperatures and the coefficients of expansion between the load sensing elements and the hydraulic cylinder rods.